SAF2507 is the third generation of super duplex stainless steel, American Standard (national standard ZG 00Cr25Ni7Mo4N). Its microstructure has an ideal duplex structure with ferrite / austenite ratio of about 50 / 50. It has the characteristics of both austenitic and ferritic stainless steel. Compared with austenitic stainless steel, SAF2507 has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, strong strength and corrosion resistance, pitting corrosion resistance Excellent crevice corrosion resistance. It is also a nickel saving stainless steel. Due to the high content of ferrite in the steel, the brittleness of the steel is increased, the fluidity is poor, the grain size is coarse, and the tendency of hot crack is greater. Moreover, due to the increase of nitrogen content, the probability of air hole in valve casting is greater. Therefore, this kind of steel brings great difficulties to the design ofand the melting of liquid steel.
At present, some domestic valve foundry manufacturers have always had some problems in the production of super duplex stainless steel SAF2507 large-diameter and high-pressure valve body. Pores, oxidation slag inclusion and cracks are serious in the production process, resulting in high scrap rate and large amount of welding repair. Therefore, the delay of construction period and the failure of delivery on schedule are caused.
(1) The super duplex stainless steel SAF2507 valve casting contains 26% chromium and more than 0.24% N, which is higher than that of ordinary duplex steel. The content of ferrite in the matrix is required to reach more than 50%, which increases many difficulties for casting production. If the content of n is not well controlled, the valve casting is easy to produce gas holes. Therefore, we use AOD refining furnace to refine the liquid steel and control the content of C, P and s off the line, Effectively control the content of n.
(2) The process riser adopts the method of multiple small risers, so it is easy to cut the riser. The pouring system adopts the bottom pouring method, and the gate adopts multiple internal gates to make the liquid level of liquid steel rise steadily during pouring, so as to achieve the balance of temperature field and the trend of sequential solidification.
(3) Control the welding repair temperature of cutting riser and valve casting, and adopt air cooling and water cooling methods to make the temperature of valve casting during cutting riser and welding repair not exceed 250 ℃.
(4) The use of the buffer plays the role of exhaust, which can reduce the gas rolled in during pouring, and can also effectively filter the liquid steel and purify the liquid steel.
According to the above process points, the batch production of duplex stainless steel SAF2507 valve castings, and the one-time film qualification rate can reach more than 85%.